1. Field of the Invention
The present invention relates to a method for manufacturing a NAND flash device, and, more specifically, to a method for forming a drain contact and a source contact in a NAND flash device.
2. Discussion of Related Art
A cell array of a NAND flash device is connected in a string unlike a common flash device. Due to this property, a drain contact and a source line contact for a global ground, which are connected by a bit line, are located at both ends of the string. These contacts are connected to the junction of a select transistor for string control.
FIG. 1 is a cross-sectional view shown to explain a method for manufacturing a flash device in a prior art.
FIG. 2 is a SEM photography for explaining conventional problems.
Referring to FIG. 1 and FIG. 2, a first interlayer insulating film 12 is formed on a semiconductor substrate 10 on which a cell string 20 for storing electrical information therein, a drain select transistor 30 for selecting a drain terminal of the cell string 20 and a source select transistor 40 for selecting a source terminal of the cell string 20 are formed.
The first interlayer insulating film 12 on a source region of the source select transistor 40 is removed by a predetermined etch process, thus forming a source line contact hole. After a polysilicon film is deposited, a chemical mechanical polishing using the first interlayer insulating film 12 as a stop layer is implemented to fill the source line contact with polysilicon, so that a source line plug 14 is formed.
A second interlayer insulating film 16 is formed. A photoresist pattern (not shown) through which a drain contact region is opened is then formed. The second and first interlayer insulating film 12 and 16 on a drain region of the drain select transistor 30 are removed by means of an etch process using the photoresist pattern as an etch mask, forming a drain contact hole. After a polysilicon film is deposited, the polysilicon film on the second interlayer insulating film 16 is removed by means of a blanket etch process. Thus, the drain contact hole is filled with polysilicon, forming a drain contact plug 18. In case of the blanket etch process, there is a danger that excessive plug poly within the contact may be lost (see “A” region in FIG. 1). Further, the blanket etch process is disadvantageous in view of process control. Excessive loss of plug poly causes a part of a nitride film for etch stop due to lack of an etch target to remain when a bit line contact and a bit line are formed by means of a subsequent damascene process. It results in degradation of electrical properties in the device (see FIG. 2).
Furthermore, one of the most critical things in the processes of forming the contacts in the NAND flash device is a formation of a drain contact. In an etch process for forming the drain contact of a NAND flash device of below 100 nm class, a margin of photoresist film is lowered. Accordingly, there is a problem that reduction in a thickness of the photoresist film is greater than reduction in a thickness of the second interlayer insulating film when the contact is etched. As described above, the drain contact plug and the source line plug are formed using independent processes. Accordingly, there is a problem that the productivity is low since a total number of masks and process steps are increased. Furthermore, it is difficult to control an overlay since each contact plug and independent mask alignment are required.